Pressure transducer

ABSTRACT

The pressure transducer provided with a measuring chamber is separated by a medium which is to be measured by a separation membrane. A piezoelectric element is arranged on the measuring chamber which is filled with silicone oil, thereby enabling the specific volume of said chamber to be modified. An adjustable path of the measuring signal is compared to a reference path and considerable deviations indicate an error on the membrane. As a result, the state of the separation membrane can be monitored when the pressure transducer is in the installed state.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority to the German application No.10342368.0, filed Sep. 12, 2003, and to the International ApplicationNo. PCT/EP2004/009863, filed Sep. 3, 2004 which are incorporated byreference herein in their entirety.

FIELD OF INVENTION

The invention relates to a pressure transducer provided with a pressuresensor located in a housing for converting a pressure to be measuredinto a measuring signal.

BACKGROUND OF INVENTION

A pressure transducer of this type, which enables the measurement of anabsolute pressure of a process medium or of a pressure difference, isknown from DE 37 05 901 C2. A pressure measuring cell with a housing isdisclosed there, in which a measuring membrane is arranged whichsubdivides a housing interior into a measuring chamber and a referencechamber. The measuring chamber and the reference chamber are eachprovided with a pressure channel which, when the pressure measuring cellis used in a difference pressure transducer, leads in each case to aseparation membrane which separates the measuring chamber and thereference chamber respectively from a measuring medium at which thepressure difference is to be measured between different measuringpoints. The two chambers are filled with a pressure transfer fluid, asilicone oil for example. In order to generate an electrical measuringsignal which changes depending on the applied pressure difference, it ispossible for the measuring membrane to carry a pressure sensor which forexample is made from silicon and is provided with elongationresistances. By using a facility for evaluating the measuring signal, itis possible to generate and output a measurement value from theelectrical measuring signal. Opposite the measuring membrane in thereference chamber is located a piezoelectric element which is providedwith electrical connections for control purposes. When energized, thispiezoelectric element serves to modulate the hydrostatic pressure in thepressure transfer fluid of the reference chamber. Rapid modulationoperations are applied in order that this increase in hydrostaticpressure takes effect in the reference chamber. The equalization ofpressure by way of the separation membrane adjacent to the pressurechannel can then be ignored. The change in pressure is transmitted tothe measuring membrane by means of the pressure transfer fluid. When theamplitude of the pressure modulation is known, the sensitivity of thepressure measuring cell is inferred directly from the amplitude of thecorresponding modulation of the measuring signal. Self-monitoring of thepressure measuring cell for malfunctions, particularly of the measuringmembrane, is thus enabled during operation and without causing anyinterruption of the measurement process. The known pressure transducerhas the disadvantage, however, that changes in or damage to theseparation membrane-cannot be identified.

SUMMARY OF INVENTION

When pressure transducers are used in process control systems it canhappen that a separation membrane is chemically attacked or mechanicallydamaged by the measuring medium. If a hole occurs in the membrane, themeasuring medium enters the measuring chamber or the reference chamberand reaches the pressure sensor which reacts sensitively to themeasuring medium. Before a total failure occurs, errored measurementvalues which the user does not notice can occur during a transitionperiod. This can have serious consequences in a process control system.In addition, as a result of the measuring medium deposits can occur on aseparation membrane which impair the transfer of pressure to the sensor.Errored measurements consequently result which are difficult for theuser to detect. Errors of this type can be recognized solely by means ofa visual inspection which requires a preceding removal of the pressuretransducer.

An object of the invention is to set down a pressure transducer whichpermits monitoring of the state of a separation membrane withoutrequiring removal of the pressure transducer.

In order to achieve this object, the new pressure transducer of the typementioned at the beginning has the features set down in the claims.Advantageous embodiments of the invention are described in the dependentclaims.

The invention has the advantage that a reliable statement concerning thestate of a separation membrane can be obtained without, for example,first having to remove the pressure transducer from a pipe line. Thediagnostics can thus be performed virtually during operation of thepressure transducer. In addition, an ageing process of the separationmembrane can be recognized and it is possible to react to this in goodtime before a failure of the pressure transducer threatens and theprocess control system in which the pressure transducer is beingoperated possibly comes to a standstill. A further advantage consists inthe fact that the measurement value delivered by a pressure transduceraccording to the invention is more reliable because changes to theseparation membrane, for example deposits or material erosion effectsresulting from corrosion or abrasion, which could corrupt themeasurement value, can be detected and reported automatically in thecase of the new pressure transducer through diagnostics. In addition,the new pressure transducer has the advantage that a possible visualinspection, which has previously been required in the case of certainmeasuring media, is dispensed with and there is thus considerably lesseffort involved in the monitoring of the state of a separation membrane.

A hole in a separation membrane can be diagnosed with a particularlyhigh degree of reliability if the value of the measuring signal, whichpresents itself within a predefinable delay period after the beginningof an essentially sudden change of volume in the measuring chamber, iscompared as the characteristic value for the path presenting itself forthe measuring signal with a corresponding characteristic value for thereference path and a signal for indicating a hole in the separationmembrane is output if the corresponding characteristic value for thereference path is not reached by-more than a predefinable extent. Thistakes advantage of the fact that a change in pressure caused by a changein volume disappears after a certain delay period as a result of aleakage in the separation membrane.

In order to reliably diagnose deposits on the separation membrane, themaximum value of the measuring signal resulting after an essentiallysudden change of volume can advantageously be compared as acharacteristic value for the path presenting itself for the measuringsignal with a corresponding characteristic value for the reference pathand a signal for indicating deposits on the separation membrane can beoutput if the corresponding characteristic value for the reference pathis exceeded by more than a predefinable extent. In analogous fashion itis advantageously possible to diagnose material erosion of theseparation membrane by monitoring whether the correspondingcharacteristic value for the reference path is not reached by more thana predefinable extent.

In an advantageous manner, preventive maintenance can be performed on apressure transducer prior to an imminent failure if a trend statement,which enables maintenance personnel to estimate when an imminent failureis to be expected, is determined and output by the evaluation unit onthe basis of timing changes in a characteristic value for the pathpresenting itself for the measuring signal in the case of temporallyspaced diagnostic operations, carried out cyclically for example.

Providing a piezoelectric element for changing the volume of themeasuring chamber, which can be controlled by the evaluation unit, hasthe advantage that there is a low power requirement for changing thevolume and the means for achieving this can be implemented with a lowresource requirement.

The functions of an evaluation unit required for monitoring a separationmembrane can also be implemented without a major resource requirement bymeans of suitable configuration of an operating program in a computerunit when an evaluation unit based on a microprocessor is used. Themanufacturing costs of the pressure transducer are thus advantageouslyhardly affected by the new diagnostic method.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention and its embodiments and advantages will be described indetail in the following with reference to the drawings in which anexemplary embodiment of the invention is represented.

In the drawings:

FIG. 1 shows a schematic diagram of a pressure transducer for absolutepressure, and

FIG. 2 shows a timing diagram with qualitative signal paths.

DETAILED DESCRIPTION OF INVENTION

FIG. 1 shows a block diagram of a pressure transducer. The pressuretransducer has an essentially axisymmetric pressure measuring cell 1with a housing 2 in which is arranged a pressure sensor 3 for convertinga pressure to be measured into an electrical measuring signal 4. Thepressure sensor 3 is situated between a measuring chamber 5 and achannel 6 which serves to deliver a reference pressure. The measuringchamber 5 is filled with silicone oil as a pressure transfer fluid. Airat the relevant current ambient pressure is delivered to the pressuresensor 3 through the channel 6. With regard to the exemplary embodimentillustrated, the measuring signal 4 thus represents the absolutepressure relative to the ambient pressure. In order to protect thesensor, the pressure transfer fluid in the measuring chamber 5 isseparated by means of a separation membrane 7 from a process medium 8which is introduced through a channel having the pressure to be measuredinto the pressure measuring cell 1. The pressure sensor 3 is protectedby the separation membrane 7 against damage by aggressive media. Athreaded stud 9 serves to facilitate installation of the pressuremeasuring cell 1 into a pipe line of a process control system which isnot shown in the drawing for the sake of clarity. The measuring signal 4is fed to a microprocessor 10 which evaluates the signal 4 in order togenerate a measurement value, outputs the measurement value on a display11 and conveys it for further processing by way of a field bus 12 to acontrol station for the process control system, which again is not shownin the figure for the sake of clarity. The microprocessor 10 is providedwith a suitable operating program for performing the measurements andthe communication. A keyboard 13 allows in puts from an operator andoutputs to the operator can take place via the display 11. In order toperform diagnostics on the separation membrane 7 a piezoelectric element14, which is to be found on the measuring chamber 5 of the pressuremeasuring cell 1 and is separated from the measuring chamber 5 by amembrane 17, can be controlled by the microprocessor 10 in such a mannerthat the volume of the measuring chamber 5 changes in accordance with anessentially predetermined timing path. In the physical sense, it is notthe size of the volume of the measuring chamber 5 that is changed by thepiezoelectric element 14, rather the volume contained within themeasuring chamber 5 is shifted such that the separation membrane 7experiences a positional change through the shift in volume. It is thusthe position of the pressure transfer fluid in the pressure measuringcell that changes. A memory 15 serves to store a path for the measuringsignal 4 which presents itself in response to the change in volume inthe case of an intact separation membrane 7. This path is recordedduring initial commissioning of the sensor in the process controlsystem. A memory 16 serves to store paths for the measuring signal 4which present themselves during subsequent operation of the pressuretransducer in response to a change in volume which is generated in orderto perform diagnostics. From the paths stored in the memories 15 and 16the microprocessor 10 determines characteristic values corresponding toone another, compares these with on e another and derives differentstatements about the state of the separation membrane 7 from the resultof the comparison. In the event of an error, a signal indicating theerror will be output on the display 11 or by way of the field bus 12 inorder that appropriate error recovery measures can be initiated by theoperations or maintenance personnel. Microprocessor 10, memory 15 andmemory 16 thus constitute components of a facility 19 for evaluating themeasuring signal 4.

As an alternative to the described exemplary embodiment, it is of coursepossible to store simply the characteristic values determined fordiagnostic purposes in the memories 15 and/or 16 instead of the completemeasuring signal paths.

Although the invention is described here with reference to an exemplaryembodiment having a pressure measuring cell for absolute pressure, it ishowever applicable in analogous fashion with regard to a measuring cellfor difference pressure.

With reference to the signal paths shown FIG. 2, the operating principleof the diagnostic process will be described in detail in the following.In the diagrams according to FIG. 2 the qualitative signal paths areshown with the time t on the abscissa and the respective signalamplitude V and U on the ordinates. Since the state of the separationmembrane 7 (FIG. 1) changes only gradually, a cyclical execution of thediagnostic process will suffice. In order to produce a change in volumeof the measuring chamber 5 (FIG. 1) according to an illustrated path 20,the piezoelectric element 14 (FIG. 1) is energized with a squarewavepulse which begins at point in time t0 and ends at point in time t1. Inresponse to this change in volume, in the case of an intact separationmembrane 7 (FIG. 1) a path 21 presents itself for the measuring signal 4(FIG. 1). At point in time to the measuring signal begins to distanceitself from its initial value S0 and gradually approaches a new finalvalue S1. The reason for this is the change in the volume under theseparation membrane 7 (FIG. 1) and the associated change in the positionof the separation membrane. As a result of the elasticity of theseparation membrane 7 this positional shift results in a change ofpressure in the measuring chamber 5 and a corresponding change in themeasuring signal 4. The invention is based on the knowledge that thebehavior of the separation membrane changes in the event of changes involume in the measuring chamber if a leakage point occurs in theseparation membrane, if deposits form on the separation membrane and/orif the thickness of the separation membrane is reduced due to abrasionor chemical reaction with the measuring medium. The path 21 is used as areference path for comparison with later measuring signal paths whichare recorded in the event of a change in volume with essentially thesame timing path 20. If a measuring signal path recorded later duringoperation deviates significantly from the reference path 21, then adefective membrane state can be inferred from this.

In the case of a hole in the separation membrane, a path 22 for themeasuring signal will present itself in response to a sudden change involume. Shortly after the point in time t0 the path 22 is still similarto the reference path 21. However, the measuring signal again headsprematurely for the initial value S0 because pressure transfer fluid canflow to the process medium through the leak in the membrane, and anequalization of pressure thus takes place. A leak in the separationmembrane can thus be reliably detected if at a later point in time, atpoint in time t2 in the exemplary embodiment illustrated, which followsthe point in time t0 by a predefined delay period, a value S2 for thepath 22 which presents itself for the measuring signal is determined iscompared with the value S1 which the reference path 21 had assumed at acorresponding point in time, and a leak is recognized and reported ifthe value S2 deviates by more than a predefined degree, in the exampleillustrated by more than 25% of the difference between the values S1 andS0. The value S2 for the path 22 lies significantly below a thresholdvalue S3 calculated in this way, with the result that a leak in themembrane is reliably recognized.

The presence of deposits on the separation membrane 7 (FIG. 1) reducesthe latter's elasticity. A change in volume of the measuring chamber 5thus results in a more marked change in pressure and a correspondinglymore marked change in the measuring signal 4 (FIG. 1) than in the caseof the reference measurement, as is represented qualitatively in FIG. 2by a path 23. In order to detect an error of this type in the separationmembrane, as the characteristic value for the path 23 the latter'smaximum value S5 is advantageously compared with the maximum value S1for the path 21 as a reference characteristic value and an error isrecognized and output if the two values deviate from one another by morethan a quarter of the difference between the values S1 and S0, in otherword s if the maximum value S5 exceeds a threshold value S4.

On the other hand the elasticity of the separation membrane is increasedin the case of material erosion, caused by abrasion or chemical reactionfor example, and the separation membrane is more easily able to follow achange in volume of the measuring chamber. In FIG. 2, a path 24 whichpresents itself in the case of a measuring membrane if the wallthickness of the membrane has already been significantly reduced bymaterial erosion effects is represented qualitatively. In similarfashion to the detection of deposits on the separation membrane, it isthus also possible with regard to material erosion effects by means of asimple comparison of a maximum value S6 for the path 24 with thethreshold value S3, which lies below the maximum value S1 for the path21 by a predefined degree, here a quarter of the difference between thevalues S1 and S0; to recognize unacceptably marked material erosion andindicate this as an error.

Since such types of changes to the separation membrane happen over anextended period of time, it is possible to derive and output a trendstatement in a simple manner when performing a cyclical execution of thedescribed diagnostic process, on the basis of the changes incharacteristic values over time, for example the maximum values of thepaths presenting themselves for the measuring signal. For example, atrend statement can specify the point in time at which deposits willexceed a level which is still reasonable. Preventive maintenance of thepressure transducer is thus possible and the additional costs that wouldbe associated with an unexpected error and its elimination can beavoided.

A change in volume according to the path 20 in FIG. 2 has proved to beparticularly advantageous. The diagnostic process can of course also becarried out with other types of paths, and characteristic valuesdeviating from the illustrated exemplary embodiment for the paths of themeasuring signal consequently presenting itself can be evaluated for thepurposes of error detection.

1. A pressure transducer, comprising: a pressure sensor arranged in ahousing for converting a measured pressure value into an electricalmeasuring signal; a measuring chamber separated from a medium to bemeasured by a separation membrane, the measuring chamber filled with apressure transfer fluid for transmitting the pressure to the pressuresensor; a piezoelectric element arranged in the measuring chamber; andan evaluation unit for evaluating the electrical measuring signal, theevaluation unit configured to: compare a characteristic valuerepresenting a timely progression of the electrical measuring signal toa reference characteristic value representing a reference timelyprogression, the timely progression of the electrical measuring signalcorresponding to a change in volume; and output a signal indicating anerror based on a difference between the characteristic value and thereference characteristic value, wherein the piezoelectric element isconfigured to be controlled such that the volume of the measuringchamber can be changed according to a desired predetermined timelyvolume progression.
 2. The pressure transducer according to claim 1,wherein the evaluation unit is configured to output a signal indicatinga leak in the separation membrane, the output signal output if a valueof the electrical measuring signal undershoots the correspondingcharacteristic value of the reference timely progression by more than afirst predetermined value, the value of the electrical measuring signalmeasured after a step-shaped volume change and after a predetermineddelay time period has elapsed upon occurrence of the step-shaped volumechange, and the measured value of the electrical measuring signal usedas the characteristic value.
 3. The pressure transducer according toclaim 1, wherein the evaluation unit is configured to output a signalindicating debris on the separation membrane, the output signal outputif a maximum value of the electrical measuring signal exceeds thecorresponding characteristic value of the reference timely progressionby more than a second predetermined value, the maximum value of theelectrical, measuring signal measured after a step-shaped changed involume, and the measured maximum value used as the characteristic value.4. The pressure transducer according to claim 1, wherein the evaluationunit is configured to output a signal indicating material erosion of theseparation membrane, the output signal output if a maximum value of theelectrical measuring signal undershoots the corresponding characteristicvalue of the reference timely progression by more than a secondpredetermined value, the maximum value of the electrical measuringsignal measured after a step-shaped changed in volume, and the measuredmaximum value used as the characteristic value.
 5. The pressuretransducer according to claim 1, wherein the evaluation unit isconfigured to output a trend statement based on a timely progression ofthe characteristic value, the timely progression of the characteristicvalue corresponding to measuring the characteristic value at discretepoints in time.
 6. The pressure transducer according to claim 1, whereinthe piezoelectric element is controlled by the evaluation unit.